This invention relates in general to a freeze drying apparatus and, more particularly, to a freeze dry flask particularly adapted for use in freeze drying and similar laboratory procedures.
Freeze drying has been found useful in many fields, such as food technology, analysis of organic materials and other uses. It is ordinarily used where the need exists to remove water or other vaporizable liquids from a substance without destroying its cellular structure. Freeze drying provides the unique capability of allowing the removal of a solvent (usually water) from a solution or suspension by sublimination. This allows the material being freeze dried to remain in a frozen, solid state until it reaches a point of dryness or the solvent is extracted. Specimens or materials are usualy freeze dried in either a chamber-type freeze dryer or a nipple-type freeze dryer, using discrete containers.
There are a number of prerequisites, however, that any vessel or specimen container must have before it may be used in freeze drying, in order to overcome the various problems encountered in this operation. One problem includes the stress created due to the sudden subjection of the container to the sub-zero temperatures necessary for quick freezing, which could result in the tendency to crack and/or shatter many materials. This stress also can result in a loss of vacuum due to improper sealing. However, the sample container must be fluid-tight in order to be satisfactory for freeze drying procedures. For the purposes of this invention, the word "fluid" will encompass both the properties of a liquid and that of a gas or vapor.
An example of the prior art over which the present invention improves is U.S. Pat. No. 3,293,773 (D. S. Frazer et al.). Previous to the subject invention, the prior art depended on a threaded cap to secure such a seal. This arrangement, if properly sealed every time, would lead to a shortened life expectancy of the sealing means involved and the possibility of vacuum leakage with the adverse consequences. Further, the differences in materials of the cap, being generally plastic or metal, and the flask, being generally glass, would cause different rates and degrees of thermal expansion and contraction, rendering the efficiency of a generally reliable seal questionable during periods of fast temperature changes.
Another example of the prior art is the cap which depended upon O-rings as a sole source of a fluid-tight seal. O-rings would be placed about the perimeter of the cap, so that, upon insertion into the mouth of the cap, a seal would be formed between the cap and the walls of the flask. For a fluid-tight seal to be possible, it is necessary that the O-rings be in complete and close-fitting contact with the flask walls at every point, and that the O-rings retain their flexibility at the low temperatures associated with freeze drying, to assure constant contact with the irregular surface of the flask walls. To maintain the constant contact and close-fitting characteristics needed, a very tight fit is necessary. This tight fit required an excessive amount of force to insert and remove the container top, often causing spillage and other losses of sample, when exerting such force. Further the friction caused by its manner of insertion, in combination with the ultra-cold temperatures of freeze-drying results in a short life expectancy of the seals.
Other problems presented in freeze drying are that the solvents evaporated from the sample may be corrosive or otherwise damaging to the specimen container. Thus, while the container is supported with a great degree of care to negate the danger of implosion or breakage, it is desirable that the flask permit an easy removal of the freeze dried product and an easy clean-up after each use.